A Fine Grind: Inspection Efficiency

Technology, experience and attitude breeds grinding success—on a very small scale.

Courtesy of All images: B. Kennedy

M&S holds tolerances down to 0.00002″ on ground parts. Shown is a secondary diameter of 0.010″ being ground on a cardiac guide wire.

An eastern Pennsylvania grinding shop is combining high-tech grinding equipment, an enthusiastic and disciplined approach to problem solving and more than 50 years of grinding experience to handle the challenges of grinding microscale parts.

In 1957, Meron Shegda picked up a used Cincinnati centerless grinder at an auction and began to take on freelance grinding jobs. In 1959, he founded M&S Centerless Grinding Inc. Beginning with a 1,200-sq.-ft. shop, the business has grown into an 11,400-sq.-ft. facility in Hatboro, Pa., employing 20 people, including 14 on the shop floor.

M&S Centerless Grinding specializes in cylindrical grinding, including OD and ID work, as well as parts chucked between centers. Meron Shegda’s son John, who began to run the company in 1990, said the M&S focus is on centerless grinding and its derivatives. About half the company’s business involves grinding microscale parts for medical applications, including implants, hypodermic tubes, device components and cardiac guide wires. A quarter of the shop’s work is what Shegda calls “ultraprecision” grinding—mostly for aerospace and high-tech customers—that involves processing parts up to 0.750″ in diameter with diameter tolerances to ±0.5µm and cylindricities to ±1µm. General grinding makes up the rest of the shop’s business, including small parts and large parts, such as 14″-dia. mining industry bushings.

“We are all over the country and the world,” Shegda said. “Many of our customers are in the Minneapolis area and in Southern California. One of the medical jobs that is running right now we are exporting to China.”

Centerless grinding involves a specific configuration of grinding wheels and workpieces. Traditional centerless grinders feature two parallel spindles placed side by side, one spinning a grinding wheel and the other a regulating wheel. The cylindrical workpiece is located between the wheels, supported by a stationary work blade. The workpiece can be fed axially between the wheels in through-feed grinding, or the grinding wheel can infeed (plunge) directly into the workpiece. Infeed grinding is necessary when a part requires a step or similar feature. Centerless grinding, especially for through-feed applications, is fast and precise, and is beneficial where workpiece flexing would be a problem when grinding between centers.

Tight and Thin

Compared to grinding larger parts, small-part and ultraprecision grinding “requires a different mentality,” Shegda said. “To go from holding a tolerance of +0.0001″/-0.000″ to +0.00005″/-0.000″ is a big leap. Then, when you look at going from 0.00005″ to 0.00002″ total tolerance, which we do, that’s an even bigger leap. You are testing the limits of the machine and what the process is able to achieve. It starts becoming about how good the equipment is and how good the people are.”

For example, in grinding pistons for a Minneapolis company, M&S holds cylindricity—roundness, taper and straightness combined—to 0.00003″. “We are holding 0.00001″ round, 0.00001″ straight and 0.00001″ taper, total deviation,” Shegda said. “It is a very high-end application. To be able to squeeze that out is very, very difficult.”

Owner John Shegda (right) confers with Tony Aoun, work center supervisor and technical sales representative.

Grinding thin-wall tubing is an M&S specialty. The company recently ground 25-gauge stainless steel hypodermic tubing, 0.030″ in diameter and 1″ long, to a wall thickness of 0.0006″. The tubes required a centerless infeed operation because the OD had a step; a small portion of the diameter remained at 0.030″, but the rest was ground to 0.025″. Although the wall was extremely thin, the ratio of diameter to wall thickness (about 47:1) was large enough that the tube was sufficiently rigid to grind. “The tube wasn’t squashing,” Shegda said. “A 0.001″-thick wall in a 0.200″-dia. tube is way less stable than a 0.0006″-thick wall at 0.025″, but a 0.0006″-thick wall is so thin that even a grain of the grinding wheel breaking loose will dent it and make the part unusable.”

M&S developed a process to consistently achieve 0.0006″ thicknesses.

“It was not just figuring out the kind of wheel and the feeds and speeds needed, but how to condition the wheel and how to work with the part that allowed us to be successful,” Shegda said.

Simply achieving a certain level of tolerance, however, doesn’t guarantee the next tighter level is possible. Shedga noted that over the course of a few months developing ways to grind the 0.0006″-thick walls, he probably spent a week trying to determine how to get from 0.0008″ to 0.0006″. “We’d try different ways to do it, but the wall would break through. The part would just blow apart in the machine if I tried to go just a little bit further with it.” When reaching the limits of what the machine and process can do, he said, “you have to get really creative to figure out ways around that.”

The creativity includes a disciplined approach—like a “science experiment,” Shegda said. Taking a pad and pen onto the shop floor, he writes down every variable in the process, changes only one variable at a time and observes the results. “We might change the truing or dressing of the grinding wheel, or we try running faster or slower or feeding harder or feeding slower. We try all kinds of different things, and look for the process to get better or worse.”

A small sewing thimble looms large over a selection of precision parts ground at M&S.

It is almost as worthwhile to have a negative result as a positive one. “If you can make some kind of change happen, even if you make something worse, you now have something that is affecting the process,” Shegda said. “If we can change that aspect and make it go a different way, it will make it better.”

The company’s longtime and recent experience expedites process development. “We’ve learned quite a bit over the last 50 years, especially the last 10, because of the different challenges that have been presented to us,” Shegda said. “Grinding that 0.0006″-thick wall, we used some techniques we recently learned in grinding plastic, and they worked.”

A True Machine

Differing machine configurations enable the shop to handle varying part requirements. M&S grinds a lot of tiny, intricate parts on CNC centerless DedTru machines from Unison. A DedTru machine basically consists of a centerless grinding fixture placed in a surface grinder. In a variation on the traditional centerless arrangement, the pair of wheels is turned 90°, putting the regulating wheel on the bottom and the grinding wheel on top. A spring-loaded pressure wheel holds the workpiece on the blade and against the regulating wheel. Then the grinding wheel is free to engage the workpiece.

Production Manager Darrin Brown at the CNC of M&S’s Royal Master GenX machine.

Shegda described a typical micropart ground on a DedTru machine: a 0.058″-long, barbell-shaped locking pin that secures a clip onto a heart valve. “The tiny pins were a big stumbling block for the device manufacturer,” he said. The pins were ground from Elgiloy Co-Cr-Ni alloy rod, heat-treated to 50 HRC. The 0.030″-dia. barbell ends had a relatively open tolerance, about ±0.0005″, but the diameter of the shaft between them was +0.0001″/ -0.0000″ (0.0191″ to 0.0190″ diameter).